Semiconductor materials are a fundamental building block of many solar power generating devices. The quality of the semiconductor structures that comprise these devices is critical to their overall power generating efficiency. Minority carrier life time and the related minority carrier diffusion length within a semiconductor structure are parameters used to characterize the quality of a semiconductor material and predict solar cell efficiency.
The number of minority carriers in a material is increased above equilibrium by external excitation (e.g., absorption of photons). The excess minority carriers decay back to the equilibrium carrier concentration by recombination of electron-hole pairs. Minority carrier lifetime is the average time that a carrier spends in an excited state within a semiconductor material before recombination. Solar cell efficiency is related to the rate at which recombination occurs. Two parameters that are indicative of the recombination rate are the minority carrier lifetime and the minority carrier diffusion length.
Techniques exist to effectively measure minority carrier lifetime of solar cells late in the manufacturing process. Specifically, after the surfaces of a solar wafer have been passivated, techniques such as quasi-steady state photoconductance (QSSPC) and microwave-detected photoconductance decay (MW-PCD) can be used to measure the minority carrier lifetime. However, wafers entering the solar manufacturing process (i.e., “as-cut” wafers) have a large density of surface defects (i.e., dangling bonds) that dominate the electron-hole recombination process. These measurement techniques are unable to distinguish recombination events in the bulk of the semiconductor material from the relatively large number of recombination events at the surface. As a result, the measurements are dominated by surface defects that will be eliminated (e.g., by gettering and passivation) later on in the manufacturing process. In addition, photo-conductivity measurements have traditionally suffered from relatively low resolution and throughput capability.
Existing photoluminescence methods can generate high resolution full wafer images, but extracting relevant parameters demonstrating good correlation to final cell efficiency is difficult for similar reasons.
There is a need for effective measurement techniques to identify the minority carrier lifetime or diffusion length early in the manufacturing process to identify semiconductor structures that will not yield suitably efficient finished solar cells. More specifically, there is a need for measurement techniques that yield results indicative of the minority carrier lifetime or diffusion length in the bulk of an “as-cut” semiconductor material.